The flaw which may be present in a transparent plate such as a glass plate includes indentations which may exist on the surface, foreign objects falling from above, and resting on and adhering to the surface, a crater-like pit and edge left by such a falling foreign object, and a foreign object and air bubble entrapped in the glass body. If the transparent plate is a glass plate whose surface is coated with a transparent film, the flaw may include a pinhole on the surface. If the flaw is accompanied with an optical defect, light incident upon the surface damaged with the flaw is abnormally refracted. A glass plate having any flaw accompanied with an optical defect must be identified via test to be rejected because such a plate is not utilizable as an optical substrate.
A testing method for detecting an optical defect of a glass plate is disclosed in the Japanese Patent Publication No. 8-247954. The testing method described in this publication consists of changing the position of a test object with respect to a light source and an image recording device, allowing the test object to pass behind a boundary edge between a light scattering portion and a light shielding portion, comparing image data acquired before the object passes behind the boundary edge and image data acquired after the object has passed behind the boundary edge, and detecting an optical defect if any based on an absolute value of the difference between the two data. However, according to this method of detecting an optical defect based on an absolute value of the difference between the two image data, one acquired before the object passes behind the boundary edge and the other acquired after the object has passed behind the boundary edge, it will not be possible to distinguish different kinds of flaws accompanied with an optical defect from one another, e.g., between a foreign object and an air bubble both entrapped in a glass plate.
Another method disclosed in the Japanese Patent No. 3178644 (U.S. Pat. No. 5691811A and EP 0726457A2) consists of allowing light from a light source to pass through a mesh structure placed close to the light source where plural thin light shielding septa and light transmitting slits repeat themselves alternately, and to impinge on a test transparent plate and penetrate the test plate, and allowing then a linearly extended camera placed opposite to the light source with the test transparent plate in between to receive light carrying an image consisting of a stripe of dark and light bars, which then transmits the image to an image processing unit so that a flaw if any in the test transparent plate can be detected through the analysis of the image. According to this method, detection of a flaw accompanied with an optical defect as distinct from a simple blemish such as a dust or soil adhered on the surface is achieved by displacing the focus of the camera apart from the mesh structure such that the black (dark) bars corresponding with the septa and white (light) bars corresponding with the slits constituting the stripe overlap with each other to give a grey strip, that is, displacing the focus of the camera to a first position where maximally bright bars and minimally bright bars merge as completely as possible to give a homogeneous bright strip where the difference in brightness between the maximally bright bands and minimally bright bands is minimized, which is called a flaw detection position, and keeping the focus at that position. Actual detection of a flaw accompanied with an optical defect consists of checking for the presence of a flag-like image signal.
Although this method allows one to distinguish a flaw accompanied with an optical defect from a simple blemish devoid of optical defect such as a dust or soil, it is impossible by this method to distinguish two different kinds of flaws both accompanied with an optical defect, e.g., to distinguish between a foreign object and an air bubble both entrapped in a test object. The method may include the use of a linearly extended mesh structure, crosswise combined mesh structures obtained by combining two linearly extended mesh structures extending in two directions crossing with each other at right angles, or a checker board patterned mesh structure obtained by arranging light shielding septa and light transmitting slits alternately in rows into a checkerboard pattern. In no matter what pattern the mesh structures may be arranged, one can not distinguish two different kinds of flaws both accompanied with an optical defect in a test object solely dependent on the pattern of light and dark spots obtained from light penetrating the structures: the same flaws may cause different patterns of light and dark spots depending, e.g., on the width of the septa or slits, or on refractions of light incident on them, or two different kinds of flaws may cause the same pattern of light and dark spots. Here, it is appropriate to assume, as an illustration, that there are, in a test transparent plate, a foreign object extending in a direction in which the plate is moved, a second foreign object and an air bubble. The first foreign object may cause a pattern comprising “light, dark, light, dark and light” spots, the second foreign object may cause a pattern comprising “dark, light, dark and light” spots, and the air bubble may cause a pattern comprising “dark, light, dark and light” spots. Thus, one cannot distinguish the three flaws from each other simply dependent on the pattern of dark and light spots carried by light passing through the mesh structure.